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Suspension Bridge

Public summary: 

Walk across our bridge, then find out what makes it stay up.

A suspension bridge made from rope and wood that children can walk across.
Useful information
Kit List: 

-one suspension bridge (two supports, two ropes with clips to suspend deck, deck, two anchors; this -should all be set up already, if not, ask a committee member)
-selection of moveable weights (small children)
-3-4m of rope
-weights on a hanger
-rope tied into a knot to have three ends coming out in different directions (optional; for vector addition)

Packing Away: 

- Unclip the decking, untie the two knots at each end of each rope (outside the pillars), but not any other knots!
- Roll up the decking tightly, and tie strongly with the loose ends of rope.
- Coil the two long ropes neatly, and put in the box along with other odd bits.
- The pillars are stored separately in the van.

Frequency of use: 

*** OVERVIEW ****

A suspension bridge made from rope and wood that children can walk across!

Possible activities:
1. Children enjoy just walking over the bridge!
2. Have one child stand on the bridge and trace the forces down to the ground. Unclip the roadway to see what holds it up. Tension.
3. Put one kid on and see how much the bridge is bent. What if we add more children?
4. Why do we want the cable to sag? Use a seperate rope with a weight in the middle. Need to pull very hard to hold up the weight with the cable horizontal, but less hard if you let the rope sag.

Other things to talk about:
1. Real life suspension bridges
2. Why build suspension bridges?
3. How to build suspension bridges - sinking piers into water, digging to bedrock, anchoring cables, etc.
3. Material science - tension and compression and cracking.

Tips for demonstrating: Don't be fazed by the crowds - keep control and take time to ask questions and demonstrate the science. If children are beginning to run over the bridge or get overexcited, limit the number of children on the bridge to one at a time.


Get children to walk over the bridge, then ask them questions. On a busy day you could stand blocking the entrance to the bridge until a group have listened to the science, then let them walk over.

What holds up the roadway? You can unclip it to show that it hangs from the sagging cables. suspension means hanging.

How does it work? Have one child stand in the middle of the bridge. He's pushing down on the roadway because he's heavy. What holds up the roadway? It's hanging from the sagging rope. Pushing and pulling forces - the rope is being pulled. Trace tension in rope over the piers and down into the ground. Get them to touch the various cables feel the different tensions. Ropes are anchored to the ground by the stakes so they have something to pull on. What would happen if we took out the stakes? Which way do the wooden piers get pushed (down into the ground).

Why do we have the rope sagging and not taught horizontal? Use a seperate rope with a weight in the middle. One child holds each end. Ask them to raise the weight into the air by pulling on the ends of the cable. Need to pull very hard to get the rope horizontal. Why? Because they're pulling sideways but they're trying to pull the weight upwards. So most of their pulling is wasted. Much easier if the rope is sagging.

Because the rope is sagging, we need these big wooden piers to hold it up. We don't want the whole roadway to sag though - cars would have to drive up to the top of the pier and down and up and down again - very silly! We want a flat roadway. So we hang the flat roadway from the sagging rope.

What about the roadway bending? Put one kid on and see how much the roadway is bent. Will the roadway be more or less bent if you add more kids all along the bridge? So the roadway doesn't bend if you spread out the load. Real suspension bridges don't bend as cars go across them! This is because the deck is usually either a beam or a truss, so can spread out the loads like when people stand all along our bendy roadway. If a train goes over the bridge then the engine is a very heavy load in one place so it will bend, so we don't tend to use suspension bridges for railways.


Real life suspension bridges. Severn bridge. Clifton bridge in Bristol. Menai and Conwy suspension bridges in Wales. Humber bridge. Union bridge (over River Tweed). Brooklyn bridge in new york. Golden Gate bridge at the entrance to San Francisco Bay.

Why build a suspension bridge rather than another type of bridge? Imagine building an arch bridge. Would need lots of bricks, which are heavy. Rope is very light and cheap, so easier to make longer bridges. Arch bridge would need lots of arches and more piers on the river bed, suspension bridge only needs two. Can be built high to allow ships to sail underneath.

Building them in real life. Steel cables not rope. Need to sig down through earth to bedrock to start building piers. May need to dig underwater using a pressurised diving chamber called a caisson - this was first done when building the Brooklyn bridge in New York. Anchor cables by dropping a heavy lump of rock on them. May have two bits of rock with interlocking teeth and cable between.

Talk about tension and compression. Rope is good under tension, brick cracks under tension. Making steel rope - wind strands of iron into a small length, wind many of them together. Testing for strength!


'Why' suspension bridges are a good idea is to do with the stability of tension - no need to waste strength and weight resisting buckling, so can make lighter and hence longer spans. Resistance to buckling is mostly a geometric effect. We really need a good demo to communicate this. This is in contrast to the strength of materials in tension and compression - most do better in compression as cracks are such a big problem in tension.

'How' they work is a matter of vector addition of forces. When explaining on tour, I used a knot with three strands of rope coming from it. That was enough for a very basic feel for what is happening, but I would really like something with springs in it to demo the actual vector addition, explain resolving into components and so on (i have an idea using some peg-board and newton-meters to do this, but making the geometry of the vector addition clear will be a real challenge).

Pedestrian suspension bridges often place the deck under tension to resist bending.

Suspension bridges aren't used for trains because they can apply such concentrated force, requiring a more concentrated deck so that a suspension bridge is no longer an efficient solution.

Risk Assessment
Date risk assesment last checked: 
Sun, 20/01/2019
Risk assesment checked by: 
Yaron Bernstein
Date risk assesment double checked: 
Mon, 04/02/2019
Risk assesment double-checked by: 
Conor Cafolla
Risk Assessment: 

Assemble bridge from sections provided with demonstrator assistance. Walk over bridge.
• Many of the risks identified become much more likely when children are playing on the
bridge in an unstructured way.
• There are actions to be taken when PUTTING UP the bridge, and actions to be
taken by the DEMONSTRATOR, but everyone involved, especially the demonstrator,
must be aware of all of these safety measures.
• Whilst in use the bridge must be supervised by a DEMONSTRATOR at all times.
Should it need to be closed and only intermittently watched, signs must be put
up at both ends. Should it need to be left completely unattended, it must be
at least partly taken down so that it isn't used.

Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Ropes Trip hazard from ropes and, especially, stakes near the ground. (Falling
onto a stake could cause a severe head injury.)
4 3 12 When PUTTING UP, mark the pegs and/or stakes with hazard tape, and, if they
are in an exposed position, cover them with boxes.
Call a first aider in the event of an accident.
3 3 9
Falling parts of the bridge Injuries from parts of the bridge falling on someone walking over it, or
near to it, should the bridge fall over.
2 4 8 The bridge must be PUT UP under the supervision of someone suitably
experienced. It must be put on level ground, and pegged down adequately (or
tied to an adequately fixed object). The piers must be attached to the rope so
that their tops can't slide. The DEMONSTRATOR must be aware that the bridge
can fall sideways if pushed hard, or swung on.
The bridge must be closed if it's fallen or partly fallen or about to fall.
Call first aider/999 if required.
1 4 4
Bridge Falls to the ground from the bridge, possibly complicated by entanglement
in the ropes.
3 4 12 The bridge should not be PUT UP on a hard surface. Grass is preferable, or
mats may be used. The DEMONSTRATOR must supervise children carefully while
crossing, and be ready to catch very small children if the parent isn't.
Control how many children are on it at a time, and do not allow jumping,
swinging or climbing on or off part way through.
Anyone tangled in the ropes may need to be untangled.
Call first aider if required.
2 3 6
Piers Banging head on the crossbars of the piers. 3 2 6 The crossbars should be covered with hazard tape when PUT UP, and more
hazard tape tied on slightly below, to make them obvious.
Call first aider if required.
2 2 4
Separate rope and weight Trip hazard from the separate rope and weight used to explain the
3 3 9 The rope and weight should be put safely out of the way - e.g. under the
bridge - when they aren't being used by the DEMONSTRATOR.
Call first aider if required.
1 3 3
Finger trap Finger-trap between planks on the deck of the bridge, and in the
2 3 6 The bridge is designed to minimize this problem. The DEMONSTRATOR should
keep children away from the underside while it's in use, and warn that
karabiners can pinch during assembly.
Call first aider if required.
2 3 6
Publicity photo: 
Experiment photos: